Multiple frequency switching power supply and methods to operate a switching power supply

ABSTRACT

A multiple frequency switching power supply including a pulse width modulation circuit having an output, having an input and having a clock signal input. The power supply also includes a switching transistor having first and second current-carrying electrodes and a control electrode. The first current-carrying electrode is coupled to a voltage source, the control electrode is coupled to the output of the pulse width modulation circuit and the second current-carrying electrode is coupled to a power supply output configured to provide a regulated output voltage. The power supply additionally includes a voltage sensing circuit coupled to the power supply output and having an output coupled to the pulse width modulation circuit input and a switch coupled to the clock input of the pulse width modulation circuit. The switch supplies a first clock signal having a first frequency when the power supply is in a normal mode of operation and supplies a second clock signal having a second frequency more than order of magnitude lower than the first frequency when the power supply is in a standby mode of operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.09/368,401 filed Aug. 04, 1999 now U.S. Pat. No. 6,127,816.

FIELD OF THE INVENTION

The invention relates to multiple frequency switching power supplies andmethods to operate a switching power supply.

BACKGROUND OF THE INVENTION

Switching power supplies are used in applications where power supplyefficiency is a concern. Several different types of switching powersupplies are known and commonly used. A first type is known as a “buck”power supply. Buck power supplies are DC-to-DC converters providing astable output voltage from an input voltage that is larger than theoutput voltage. In other words, by bucking a portion of theelectromotive force of the input voltage, the buck supply is able toprovide a regulated, reduced output voltage. Ideally, the buck powersupply is able to perform voltage step-down and regulation functionswith very little power loss.

A second type of switching power supply is known as a “boosting” powersupply. Boosting power supplies incorporate a transformer or coil andone or more switches coupled in series with a primary winding of thetransformer or in series with the coil. Boosting power supplies are ableto supply a DC output voltage from a DC input voltage that is lower thanthe output voltage. Examples of switching power supplies using bothprinciples of operation are described in U.S. Pat. No. 5,691,632,entitled “Switching Power Supply,” issued to Otake and herebyincorporated herein by reference. These examples use a switchingtransistor as a synchronous rectifier. Efficiency of switchingtransistor operation is improved by reducing charge storage effects.

Other examples of switching power supplies are known. For example, U.S.Pat. No. 5,675,479, issued to Tani et al. and hereby incorporated hereinby reference, discloses a switching power supply where efficiency isimproved under light loading by lowering the switching speed of aswitching element. Under heavy load, output ripple is reduced byincreasing switching speed.

U.S. Pat. No. 5,390,101, issued to Brown and hereby incorporated hereinby reference, discloses a switching power supply having a voltagecontrolled oscillator (VCO) to provide high efficiency operationthroughout a wide range of input voltages and load conditions. VCOfrequency is increased when output loading increases. U.S. Pat. No.4,683,529, issued to Bucher II and hereby incorporated herein byreference, discloses a switching power supply that uses pulse-widthmodulation together with switching frequency modulation to maintain highefficiency.

All of these examples are concerned with maintaining efficiency overportions of a load curve where significant amounts of power are beingdrawn from the switching power supply. As a result, the range offrequencies over which they operate is relatively narrow. Additionally,power dissipation in switching power supplies (i.e., inefficiency) iscomposed of two principal components: (i) conduction losses, caused byparasitic resistance in power supply components and (ii) switchinglosses, caused by charge storage and other effects in the switchingelements. Switching losses are proportional to switching speed.

When the amount of electrical power being drawn from the switching powersupply is reduced to very low levels, the conduction losses become verysmall and the switching losses are the dominant source of switchingpower supply inefficiency. For example, switching power supplies thathave efficiencies on the order of 95% under normal loading may haveefficiencies of about 50% under standby conditions, due primarily toswitching losses.

Moreover, increased concern over pollution caused by power generationand increasingly larger numbers of electrically-powered devices used inhomes and industry combine to create new standards and guidelines forpower consumption budgets for electrical appliances. Also, increasingnumbers of electrical appliances are maintained in a “ready-to-operate”state twenty-four hours a day.

Further, the United States Government Environmental Protection Agency(EPA) has developed new guidelines for compliance with Energy Star powerconsumption limit guidelines for power budgets for such appliances. As aresult, these appliances are being designed to incorporate power-saving“standby” modes whereby the appliance is both ready to be operated andis consuming as little electricity as possible whilst in the standbymode.

Additionally, as the number and the diversity of electrically-poweredappliances has increased, especially data communications, data storageand data manipulation appliances, demand has grown for battery-poweredelectrical appliances. Consumer trends for such appliances place heavyemphasis on size and weight for the appliances. A particular emphasis onincreased battery life, and hence on reduced consumption of electricalpower, places a substantial premium on reduction of power consumption insuch appliances.

What is needed is a new type of switching power supply capable ofextremely low power consumption in a standby mode while still beingcapable of supporting all required functions in a host appliance andwhich is also capable of switching very rapidly to full power operationon demand.

SUMMARY OF THE INVENTION

In a first aspect, the invention provides a multiple frequency switchingpower supply. The power supply includes a pulse width modulation circuithaving an output, having an input and having a clock signal input. Thepower supply also includes a switching transistor having first andsecond current-carrying electrodes and having a control electrode. Thefirst current-carrying electrode is coupled to a voltage source, thecontrol electrode is coupled to the output of the pulse width modulationcircuit and the second current-carrying electrode is coupled to a powersupply output configured to provide a regulated output voltage. Thepower supply additionally includes a voltage sensing circuit coupled tothe power supply output and having an output coupled to the pulse widthmodulation circuit input and also includes a switch coupled to the clockinput of the pulse width modulation circuit. The switch supplies a firstclock signal having a first frequency to the pulse width modulationcircuit when the power supply is in a normal mode of operation andsupplies a second clock signal having a second frequency more than orderof magnitude lower than the first frequency to the pulse widthmodulation circuit when the power supply is in a standby mode ofoperation.

In another aspect, the invention provides a method to operate aswitching power supply. The method includes determining when an outputcurrent from the power supply falls below a threshold and switching fromone switching frequency to another, discrete switching frequency whenthe output current falls below the threshold.

In another aspect, the invention provides a method to operate aswitching power supply. The method includes determining when an outputcurrent from the power supply is above a threshold and supplying a firstswitching signal having a first frequency to a switching transistor inthe power supply when the output current is above the threshold. Themethod also includes determining when the output current from the powersupply is below the threshold and supplying a second switching signalhaving a second frequency to the switching transistor in the powersupply when the output current is below the threshold. The secondfrequency is less than one tenth of the first frequency.

In another aspect, the invention includes provision of a control inputon a multiple frequency switching power supply. Commands from a printengine controller select a first switching frequency for the powersupply when the printer is in a normal mode of operation and select asecond switching frequency for the power supply when the printer is in astandby mode of operation.

DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a simplified block diagram of a printer incorporating amultiple frequency switching power supply, in accordance with anembodiment of the present invention.

FIG. 2 is a simplified schematic diagram of a multiple frequencyswitching power supply, in accordance with an embodiment of the presentinvention.

FIG. 3 is a simplified schematic diagram of a multiple frequencyswitching power supply, in accordance with an embodiment of the presentinvention.

FIG. 4 is a simplified schematic diagram of a multiple frequencyswitching power supply, in accordance with an embodiment of the presentinvention.

FIG. 5 is a simplified schematic diagram of a multiple frequencyswitching power supply, in accordance with an embodiment of the presentinvention.

FIG. 6 is a simplified block diagram of a group of power supplies forthe laser printer of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified block diagram of a laser printer 10 incorporatinga multiple frequency switching power supply 12, in accordance with anembodiment of the present invention. The laser printer 10 includes adata input port 14, a data memory 16, a controller 18 and a print engine20. The laser printer 10 also includes a power consumption monitor 22, aprimary AC-to-DC power supply 24 and one or more efficient DC-to-DCswitching power supplies 12 coupled to the primary power supply 24.

Laser printers 10 may be formed to provide relatively high speed printcapability, as described, for example, in U.S. Pat. No. 5,195,176,entitled “Method and Apparatus to Enhance Laser Printer Speed andFunctionality,” issued to Lung and hereby incorporated herein byreference. Color print capability is described in U.S. Pat. No.5,018,805, entitled “Laser Printer,” issued to Kessler and herebyincorporated herein by reference. Improvements in or affecting printengines in laser printers are described in U.S. Pat. No. 5,245,442,entitled “Multi-Functional Laser Printer,” issued to Yang and U.S. Pat.No. 5,079,569, entitled “Laser Printer With Paper Positioning andTensioning Features,” issued to Bunch, Jr., which patents are herebyincorporated herein by reference.

In laser printers 10, the data input port 14 and the data memory 16 aretypically digital circuits that operate together under the direction ofthe controller 18 to accept input data at a relatively high speed at theinput port 14, store the input data in the data memory 16 and thenprocess portions of the input data to provide printed output matter fromthe print engine 20.

In laser printers 10, output data, in the form of printed material, isoutput from the print engine 20 at a much lower rate than the data inputrate. When the laser printer 10 has input data supplied to the datainput port 14, the data memory 16 provides temporary storage of theinput data until the input data can all be processed.

In order to reduce power consumption due to operation of the data inputport 14, the data memory 16, the controller 18 and the print engine 20,these components have been designed to operate with progressively lowerpower supply voltages over the last several years. However, other kindsof functions, such as an electromechanical drive for feeding paperthrough the print engine 20, require higher voltages or currents inorder to operate properly. As a result, the AC-to-DC power supply 24 isoften used to provide a primary power supply, with one or more efficientDC-to-DC switching power supplies 12 to provide the data-handlingcircuits 14, 16 and 18 with electrical power.

In normal operation, the power consumption monitor 22 detects normalpower consumption and provides control signals to maintain normal powersupply functions. When, however, no input data are present at the datainput port 14 and the print engine 20 is not active, power consumptiondecreases. When the power consumption monitor 22 detects reduced powerconsumption, signals from the power consumption monitor 22 place themultiple frequency switching power supply 12 in a standby mode.

In the standby mode, the multiple frequency switching power supply 12switches from a first clock signal to a second clock signal having alower frequency than the first clock signal to reduce switching speedand thus to reduce switching losses in the multiple frequency switchingpower supply 12. In one embodiment, the second clock signal has afrequency that is one-tenth to one-one hundredth of the frequency of thefirst clock signal. In the standby mode, the amount of electrical powerbeing drawn from the power supply in the standby mode may be more than afactor of ten less than is drawn in the normal operating mode. Forexample, the power draw in the normal operating mode might be 200 Watts,with only 10 Watts being required in the standby mode.

There are significant advantages to discontinuously switching to asecond clock signal having a frequency that is well outside of a rangeof frequencies for a first clock signal. A first advantage is that asharply reduced clock signal frequency also provides a sharp increase inswitching power supply efficiency because switching losses aredramatically reduced. A second advantage is that circuit complexity isreduced compared to what would be required for a single oscillator to beable to provide such diverse frequencies.

A third advantage is that the range of clock frequencies availablethrough discontinuous switching of clock frequencies is increased. Thisarises because the small changes in loading that are used to provideincremental or continuous, rather than discontinuous, changes in clocksignal frequency do not support such great modification of clockfrequency from a VCO.

VCOs operate over a limited range of frequencies having an upperfrequency bound that is typically only two to three times as great as alower frequency bound. In other words, a switching power supply thatuses a VCO to vary switching frequency within a range of loads typicallycannot encompass a wide enough frequency range to obtain the reductionin switching frequency that is needed in order to provide greatlyincreased efficiency in the standby mode.

FIG. 2 is a simplified schematic diagram of a multiple frequency (MF)switching power supply 30, in accordance with an embodiment of thepresent invention. The MF switching power supply 30 may be used for themultiple frequency switching power supply 12 of FIG. 1. While the MFswitching power supplies shown in FIGS. 2-5 are shown as being buckpower supplies, it will be appreciated that other kinds of switchingpower supplies may use the inventive concepts illustrated, including“boost”, “fly back” and “forward converter” types of switching powersupplies.

The MF switching power supply 30 includes a power input port 32 toaccept an input voltage V_(IN), a switching transistor 34, a filteringcircuit 36, a voltage divider 37, a power output port 38 to provide anoutput voltage V_(OUT) and a pulse width modulation (PWM) circuit 40having an input 42. The MF switching power supply 30 also includes aswitch 44 and multiple clock signal sources or oscillators 46 and 48.

The PWM circuit 40 input 42 may be switched by the switch 44 toselectively couple one of several oscillators 46 and 48 to the PWMcircuit 40 in response to control signals from the power consumptionmonitor 22 of FIG. 1. When the power consumption monitor 22 detectsreduced power consumption, the power consumption monitor 22 causes theswitch 44 to switch from a first clock signal from a first oscillator 46having a first clock frequency f_(HI) to a second clock signal from asecond oscillator 48 having a second clock frequency f_(LO), reducingthe clock frequency, and thus the switching frequency of the switchingtransistor 34, in the standby mode. In one embodiment, the switchingfrequency is reduced by one or more orders of magnitude in the standbymode.

When the power consumption monitor 22 detects increased powerconsumption, the power consumption monitor 22 causes the switch 44 toswitch from the second oscillator 48 to the first oscillator 46,increasing the output current from the MF switching power supply 30 andrestoring the laser printer 10 of FIG. 1 to normal operation. In oneembodiment, the first clock frequency f_(HI) is 100 kilohertz and thesecond clock frequency f_(LO) is 1 kilohertz.

It will be appreciated that the frequency f_(HI) of the first clocksignal may be varied in a continuous fashion in order to operate themultiple frequency switching power supply 12 at high efficiency over arange of loads in the normal mode. A VCO may be used for the firstoscillator 46 to allow the clock frequency f_(HI) to increase as thecurrent output from the multiple frequency switching power supply 12 isincreased.

For example, the first oscillator 46 may sense power supply loadingusing the voltage divider 37, as indicated by a dashed line in FIG. 2.Variation of switching power supply operating frequency to maintain highefficiency over a range of loads is discussed in U.S. Pat. No.5,691,632, issued to Otake and hereby incorporated herein by reference.

The MF switching power supply 30 operates by turning the switchingtransistor 34 ON and OFF to supply electrical charge from the powerinput port 32 to the filtering circuit 36. The resistive voltage divider37 is formed from two resistors R₁ and R₂ and provides a node at ajunction of the two resistors R₁ and R₂ for one input of the PWM circuit40. A gate of the switching transistor 34 is coupled to an output of thePWM circuit 40. A pulse width of the ON portion of the duty cycle of theswitching transistor 34 is varied in a conventional manner by the PWMcircuit 40 in response to voltages sensed by the voltage divider 37 toregulate the amount of charge that is transferred per cycle from thepower input port 32 to the filtering circuit 36 and thus to maintain thedesired output voltage V_(OUT) at the output 38. In one embodiment, thelimits over which the duty cycle may be varied in the PWM circuit 40 aredifferent when the PWM circuit 40 is driven by the first oscillator 46than when the PWM circuit 40 is driven by the second oscillator 48. Thiscan provide hysteresis to reduce or avoid excessive switching betweenthe normal operating mode and the standby mode.

The filtering circuit 36 acts to smooth out pulses from the switchingtransistor 34. In one embodiment, the filtering circuit 36 includes aseries-connected inductor L₁ and a shunt-connected capacitor C₁. A diodeD₁ acts as a clamp to prevent transient changes in current through theinductor L₁ from driving the side of the inductor L₁ that is coupled tothe switching transistor 34 significantly below ground.

In one embodiment, the MF switching power supply 30 includes a ripplemonitor 49. The amount of ripple that is present in the output voltageV₀ tends to increase as the amount of power being drawn from the MFswitching power supply 30 increases. When the ripple monitor 49determines that the ripple in the output voltage V₀ is too high, theripple monitor 49 provides a signal to the controller 18 of FIG. 1 toincrease the frequency of the MF switching power supply 30. In oneembodiment, the frequency is increased by switching to the firstoscillator 46. In one embodiment, the frequency of the first oscillator46 is increased when the ripple monitor 49 detects unacceptably highripple, for example by changing a control voltage controlling a VCOemployed for the first oscillator 46.

FIG. 3 is a simplified schematic diagram of a multiple frequencyswitching power supply 50, in accordance with an embodiment of thepresent invention. Many elements used in the MF switching power supply50 are identical to elements used in the MF switching power supply 30 ofFIG. 2. These elements are given the same reference numbers as are usedin FIG. 2 and explanation of these elements will not be repeated. Thedashed line coupling the first oscillator 46 (f_(HI)) to the voltagedivider 37 of FIG. 2 has been eliminated from subsequent Figures forclarity of illustration.

The MF switching power supply 50 includes a modified filtering circuit36′. The modified filtering circuit 36′ includes three additionalresistors, R_(S), R₃ and R₄. The resistor R_(S) is a low value resistorcoupled between the inductor L₁ and the capacitor C₁ that develops asmall voltage proportional to the current being drawn from the MFswitching power supply 50. A first voltage divider 52 is formed byresistors R₁ and R₂ coupled in series between a first end of theresistor R_(S) and ground, and a second voltage divider 54 formed byresistors R₃ and R₄ coupled between a second end of the resistor R_(S)and ground.

A voltage difference between voltages developed in the first voltagedivider 52 and the second voltage divider 54 is sensed by a currentsensing circuit 56. The current sensing circuit 56, together with thecurrent sensing resistor R_(S) and the first and second voltage dividers52 and 54, is one embodiment of the power consumption monitor 22 of FIG.1.

In one embodiment, the current sensing circuit 56 includes a firstoperational amplifier 57, a resistor 58 and a second operationalamplifier 59. The first operational amplifier 57 has a non-invertinginput coupled to a node joining the resistors R₁ and R₂ forming thefirst voltage divider 52. The first operational amplifier 57 has aninverting input coupled to a node joining the resistors R₃ and R₄forming the second voltage divider 54. The resistor 58 sets a gain forthe first operational amplifier 57.

An output signal from the first operational amplifier 57 is coupled toan inverting input to the second operational amplifier 59. A referencevoltage V_(REF) is coupled to a non-inverting input to the secondoperational amplifier 59. The reference voltage V_(REF), together withthe resistor 58, the resistor R_(S) and the first and second voltagedividers 52 and 54, determine a current level at which the currentsensing circuit 56 switches from a normal operating state to a standbystate and vice versa. The output signal from the first operationalamplifier 57 may optionally be filtered using an R-C filter 60 includinga series resistor R and a shunt capacitor C, as shown in FIG. 3.

An output of the second operational amplifier 59 is coupled to theswitch 44. When the current sensing circuit 56 detects that the currentbeing drawn from the output 38 has dropped below a threshold level, theoutput from the second operational amplifier 59 switches the PWM 40 fromthe first oscillator 46 providing the first clock signal (f_(HI)) to thesecond oscillator 48 providing the second clock signal (f_(LO)). Thisputs the MF switching power supply 50 into the standby mode. When thecurrent sensing circuit 56 detects that the current being drawn from theoutput 38 has increased above the threshold, the second operationalamplifier 59 changes state, switching the switch 44 to provide the firstclock signal (f_(HI)). This puts the MF switching power supply 50 backinto the normal operating mode from the standby mode.

FIG. 4 is a simplified schematic diagram of a multiple frequencyswitching power supply 70, in accordance with an embodiment of thepresent invention. The MF switching power supply 70 includes a dutycycle sensing circuit 72 having an input coupled to the gate of theswitching transistor 34 and having an output coupled to the switch 44.

When the duty cycle sensing circuit 72 detects that the duty cycle ofthe switching transistor 34 falls below a threshold, the duty cyclesensing circuit 72 causes the switch 44 to couple the second oscillator48 supplying the second clock signal (f_(LO)) to the PWM 40. This setsthe MF switching power supply 70 to the standby mode of operation.

When the duty cycle sensing circuit 72 detects need for increased poweroutput from the MF switching power supply 70, for example due toincreased duty cycle of the switching transistor 34, the duty cyclesensing circuit 72 switches the MF switching power supply 70 from thestandby mode to the normal mode by coupling the first oscillator 46 tothe PWM 40 to provide the first clock signal (f_(HI)) to the PWM 40.

FIG. 5 is a simplified schematic diagram of a multiple frequency (MF)switching power supply 74, in accordance with an embodiment of thepresent invention. In the embodiment shown in FIG. 5, the MF switchingpower supply 74 switches from the normal mode of operation using thefirst oscillator 46 having the first clock frequency f_(HI) to thestandby mode of operation using the second clock signal from the secondoscillator 48 having the second clock frequency f_(LO) in response tocommands from the controller 18 of FIG. 1. This permits the controller18 to cause more than one MF switching power supply, such as the powersupply 74, to enter the standby mode when the laser printer 10 of FIG. 1enters the standby mode.

FIG. 6 is a simplified block diagram of a group of power supplies 80 forthe laser printer 10 of FIG. 1. Modern laser printers 10 include anumber of power supplies 80, many or all of which may be switching powersupplies. The power supplies 80 include a filter 82 that couples ACinput power to an active power factor control power supply 84. In oneembodiment, the active power factor control power supply 84 converts 120volts AC to 400 volts DC.

An output from the active power factor control power supply 84 iscoupled to a capacitor 86 and to an input to a fly back converter 88. Anoutput from the fly back converter 88 is coupled to a primary winding ofa transformer 90. A secondary winding of the transformer 90 is coupledthrough a diode 92 to a capacitor 94 to provide a +24 volt power source.A +5 volt buck converter 96 is coupled to the +24 volt power source toprovide a +5 volt output, and a +3.3 volt buck converter 98 is coupledto the +5 volt output to provide a +3.3 volt output.

The controller 18 is able to couple either a first oscillator 100 havinga first frequency f_(HI) or a second oscillator 102 having a secondfrequency f_(LO) to the active power factor power supply 84 via a switch104 in response to signals from the power monitor 22 of FIG. 1 or inresponse to other assessments of power need. The controller 18 also isable to couple either a first oscillator 106 having a first frequencyf_(HI) or a second oscillator 108 having a second frequency f_(LO) tothe fly back converter 88 via a switch 110. The controller 18 also isable to couple either a first oscillator 112 having a first frequencyf_(HI) or a second oscillator 114 having a second frequency f_(LO) tothe +5 volt buck converter 96 via a switch 116. The controller 18 alsois able to couple either a first oscillator 118 having a first frequencyf_(HI) or a second oscillator 120 having a second frequency f_(LO) tothe +3.3 volt buck converter 98 via a switch 122. The first frequencyf_(HI) and the second frequency f_(LO) need not be the same for each ofthe power supplies 84, 88, 96 and 98. The controller 18 may switch allof the power supplies 84, 88, 96 and 98 together or may switch them in apreferred order, or may set them individually in response to powerlevels being drawn from each of them.

In the embodiments of FIGS. 2 through 6, a small amount of hysteresismay be included to reduce or prevent unnecessary switching between thetwo frequencies f_(HI) and f_(LO). The hysteresis may be provided bysimply including a short delay, or by setting thresholds for switchingfrom one state to be slightly different than thresholds for switchingback to the one state or may be implemented in other ways.

It will be appreciated that the switching transistor 34 of FIGS. 2through 5 may be a MOS power transistor (as shown) or may be a bipolarpower switching transistor. Other types of switching elements that maybe pulse width modulated may also be employed.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A multiple frequency switching power supplycomprising: a pulse width modulation circuit having an output, having aninput and having a clock signal input, the pulse width modulator beingconfigured to vary width of an output pulse in response to an amount ofpower needed from the power supply output; a switching transistor havingfirst and second current-carrying electrodes and having a controlelectrode, the first current-carrying electrode being coupled to avoltage source, the control electrode being coupled to the output of thepulse width modulation circuit, the second current-carrying electrodebeing coupled to a power supply output; and a switch coupled to theclock input of the pulse width modulation circuit, the switch supplyinga first clock signal having a first frequency when the power supply isin a normal mode of operation and supplying a second clock signal havinga second frequency more than order of magnitude lower than the firstfrequency when the power supply is in a standby mode of operation. 2.The power supply of claim 1, wherein the first frequency is in a band offrequencies and the second frequency is outside the band of frequencies.3. The power supply of claim 1, further comprising a voltage sensingcircuit coupled to the power supply output and having an output coupledto the pulse width modulation circuit input, the first frequency isderived from a voltage controlled oscillator responsive to the voltagesensing circuit, the voltage controlled oscillator providing a firstclock signal that has a frequency proportional to the output current. 4.The power supply of claim 1, wherein the voltage sensing circuitincludes a resistive voltage divider.
 5. The power supply of claim 1,wherein the power supply is a buck power supply and further comprises afiltering circuit having an input coupled to the second current-carryingelectrode of the switching transistor and having an output coupled tothe power supply output and configured to provide a regulated outputvoltage.
 6. The power supply of claim 5, wherein the filtering circuitincludes: an inductor having a first lead coupled to the second currentcarrying electrode of the switching transistor and having a second leadcoupled to the output; and a capacitor having a first lead coupled tothe output and having a second lead coupled to ground.
 7. The powersupply of claim 1, wherein the first frequency is several orders ofmagnitude greater than the second frequency.
 8. A multiple frequencyswitching power supply comprising: a pulse width modulation circuithaving an output, having an input and having a clock signal input; aswitching transistor having current-carrying electrodes coupled to avoltage source and to a power supply output to provide a regulatedoutput voltage, respectively, and having a control electrode coupled tothe output of the pulse width modulation circuit; and a switch coupledto the clock input of the pulse width modulation circuit, the switchsupplying a first clock signal having a first frequency when the powersupply is in a normal mode of operation and supplying a second clocksignal having a second frequency when the power supply is in a standbymode of operation.
 9. The power supply of claim 8, wherein the switchtoggles from supplying the first clock signal to supplying the secondclock signal and vice versa in response to signals from circuitryexternal to the power supply.
 10. The power supply of claim 8, whereinthe switch toggles from supplying the first clock signal to supplyingthe second clock signal and vice versa in response to signals from apower consumption monitor internal to the power supply.
 11. The powersupply of claim 8, further comprising a voltage sensing circuit coupledto the power supply output and having an output coupled to the pulsewidth modulation circuit input, the voltage sensing circuit and thepulse width modulation circuit being configured to cause a frequency ofthe first signal to increase in response to more current being drawnfrom the power supply output.
 12. The power supply of claim 8, furthercomprising a power consumption monitor providing signals to the switchto toggle the switch comprising: a first voltage divider having an inputcoupled to the second current-carrying electrode and having a firstoutput; a current sensing resistor having a first electrode coupled tothe second current carrying electrode and a second electrode coupled tothe power supply output; a second voltage divider having an inputcoupled to the power supply output and having a second output; a firstdifferential amplifier having a first input, a second input and anoutput, the first input being coupled to the first voltage divideroutput, the second input being coupled to the second voltage divideroutput; and a second differential amplifier having a first input, asecond input and an output, the first input being coupled to the firstamplifier output, the second input being coupled to a reference voltagesource and the output being coupled to the switch, the seconddifferential amplifier providing a first voltage to the switch when anoutput current from the power supply is above a threshold and providinga second voltage to the switch when the power supply voltage is belowthe threshold.
 13. The power supply of claim 8, further comprising aduty cycle detection circuit having an input and having an output, theinput being coupled to the output of the pulse width modulation circuit,the output being coupled to the switch and causing the switch to couplethe first clock signal to the pulse width modulation circuit when theduty cycle exceeds a threshold and causing the switch to couple thesecond clock signal to the pulse width modulation circuit when the dutycycle does not exceed the threshold.
 14. The power supply of claim 8,wherein the first clock signal has a frequency that is related to amagnitude of a current being drawn from the power supply output.